[RobRich]

Also check here:

http://www.sysopt.com/articles/soi/

SOI is very important in regards to capacitance.

The diagram above represents a MOS circuit (left) as compared to an SOI circuit (right). The MOS circuit is slower because a capacitance develops between the impurity layers and the silicon substrate. This boundary area is known as the junction capacitance layer. As noted earlier, a MOS circuit must spend a large amount of its operational time discharging and recharging this capacitance. In comparison, the SOI circuit employes silicon oxide layered over the pure silicon substrate. The SOI circuit's capacitance will be negligible since the silicon oxide provides an efficient insulation barrier. The junction capacitance area is eliminated by SOI, thus the transistor will be able to operate faster since the charging process is eliminated.